The present invention generally relates to implantable medical devices.
There are many kinds of implantable medical devices. Some monitor patient conditions while others disperse some form of therapy. One particular type of implantable medical device is an implantable cardiac therapy device, or ICTD. ICTDs are implanted within the body of a patient to monitor, regulate, and/or correct heart activity. ICTDs include implantable cardiac stimulation devices (e.g., implantable cardiac pacemakers, implantable defibrillators) that apply stimulation therapy to the heart as well as implantable cardiac monitors that monitor heart activity.
Implanted medical devices are typically capable of being programmed remotely by an external programming device, often called a xe2x80x9cprogrammerxe2x80x9d. Individual ICTDs, for example, are equipped with telemetry circuits that communicate with the programmer. One type of programmer utilizes an electromagnetic wand that is placed near the implanted cardiac device to communicate with the implanted device. When used in a sterile field, the wand may be enclosed in a sterile sheath. The wand contains a coil that forms a transformer coupling with the ICTD telemetry circuitry. The wand transmits low frequency signals by varying coil impedance.
Early telemetry systems were passive, meaning that the communication was unidirectional from the programmer to the implanted device. Passive telemetry allowed a treating physician to download instructions to the implanted device following implantation. Due to power and size constraints, early commercial versions of implanted devices were incapable of transmitting information back to the programmer. As power capabilities improved, active telemetry became feasible, allowing synchronous bi-directional communication between the implanted device and the programmer. With improved processor and memory technologies, the implanted devices have become increasingly more sophisticated, allowing them to monitor many types of conditions, store the conditions and upload them to the programmer, and apply tailored therapies in response.
Current telemetry systems have a limited communication range between the programmer wand and the ICTD, which is often referred to as xe2x80x9cshort-range telemetryxe2x80x9d or xe2x80x9cwand telemetryxe2x80x9d. For effective communication, the wand is held within two feet of the ICTD, and more typically within several inches. One limitation is that the ICTD lacks sufficient power to transmit longer range signals. Another limitation is the slow speed at which data is transferred. Data is commonly transferred at less than one KByte/second. Holding a wand in place or strapping it to a patient for a minute or more is not considered expedient for medical personnel.
These shortcomingsxe2x80x94short transmission distance and limited speedxe2x80x94can be overcome by adoption of radio frequency techniques of communication. Although being an established medium of communication, use of radio frequency techniques is not as easy as it might seem. One problem is that RF energy is not easily transmitted out of a patient""s body. The patient""s body tissue inhibits transmission by absorption of the emitted energy. The radio transmitter further requires an antenna having at least one dimension that is a significant fraction of a wavelength of the electromagnetic energy being emitted. Since size matters for implanted devices, the antenna size and structure is a significant factor to consider.
Furthermore, the implanted device is hermetically sealed in a metal enclosure or xe2x80x9ccanxe2x80x9d that prevents penetration of electromagnetic energy of high frequency. The metal can limits communication to the low frequency range of less than 200 KHz. In one exemplary system, signals sent from the programmer to the implanted device are transmitted at approximately 36 KHz, and data is transmitted back from the implanted device to the programmer at approximately 8 KHz.
Accordingly, there is an ongoing need to improve the communication capabilities between implanted devices and external devices, and particularly the need to communicate more effectively over greater transmissions ranges.
An implantable medical device is equipped with a magnetic coupling assembly to transfer communication signals to and from the device using electromagnetic energy. Two representative implementations are disclosed: a torid-coupled lead antenna and an inductive feed-through. The assemblies transfer electromagnetic energy between internal device circuitry and an external antenna (e.g., a lead or dedicated antenna). The assemblies effectively extract high-frequency data from the antenna while preventing high-frequency emanations from disrupting the internal device circuitry.